Glassy magnetic behaviors in the single layered cobaltite Gd0.8Sr1.2CoO4

•The Gd0.8Sr1.2CoO4 specimen exhibits only one spin glass state at fairly low temperature (below 9K).•The aging effect and long-time relaxation phenomenon are observed in Gd0.8Sr1.2CoO4.•The large structural distortion and the stability of the lower spin state in cobalt ions are the microscopic origin for such spin glass.